It has long been attempted to economically exploit oil shale as a source of shale oil. Basically, at temperatures above 900.degree. F., a disproportionation of carbon and hydrogen structures in kerogen occurs. The solid high molecular weight kerogen is broken down and converted into a distillable hydrocarbonaceous liquid oil fraction with a lower carbon to hydrogen ratio and other fractions. The volatilized oily distillate can be recovered as a useful distillate.
In a conventional retort, the yield of the oil is in the range of about 60-70% of the organic matter in the raw shale. An additional 7 to 10% is converted into light gases and about 20 to 25% into a carbon rich residue retained in the organic material called char. As described in Kimberlin et al., U.S. Pat. No. 2,966,450, shale oil is characterized by a high nitrogen content, low carbon to hydrogen ratio, and high sulfur content.
U.S. Pat. No. 2,694,037 (Johnson et al.) describes a method and apparatus for the production of shale oil from oil shale. Oil-bearing material, particularly oil shale, is crushed, preheated by contact with the gaseous product from a downstream cracking reactor, and then passed to a retorting zone wherein the shale bed is passed downwardly countercurrently to the rising stream of hot gases so as to separate the oil from the oil-bearing material. Combustion occurs in the lower section of the retort, sustained by oxygen enriched air, preferably approximately pure oxygen. Close to the zone of combustion and directly beneath it is located means for introducing and distributing water used to cool hot spent shale coming from the zone of combustion and thereby supply steam required for the process and control of the volume percentage of oxygen in the zone of combustion. The steam is superheated as it passes upwardly through the combustion zone and acts with the combustion gases to supply heat necessary to drive the oil from the oil shale. Vaporized shale oil is quenched and cooled in a quench tower and fractionated. Johnson et al further disclose a helical screw-type conveyor for removing fresh shale to a mixer or other treating apparatus.
U.S. Pat. No. 4,058,205 (Reed) discloses the production of crude shale oil by a process wherein crushed oil shale is mixed with small quantities of coal, preheated, and retorted in a horizontal chamber by the indirect heat obtained when the residual char and coal mixture is burned in a furnace. Reed indicates that in situ retorting, where shale is burned in place and the heat produced decomposes the surrounding shale, has been largely unsuccessful because of the impermeability of the shale which prevents movement of the gases including both the air required for combustion and the product vapors. Also known, as indicated in Reed, is direct combustion retorting where crushed shale is heated by combustion occurring in the retort by burning injected fuels and/or residual carbon remaining in the retorted shale. This is done in a vertical vessel where fresh shale is fed into the top of the vessel and spent shale is removed from the bottom. Air for combustion is forced into the bottom where combustion occurs. The hot gasses pass up through the shale causing the kerogen to decompose. The product is removed as a vapor out of the top and condensed. The design is described as having the advantage of good heat efficiency, but the disadvantage of dilution of the retort gases with combustion gases.
Considering now the various apparatus that have been devised for the extraction of oil from oil shale, U.S. Pat. Nos. 52,283 and 52,284 (Gengembre) disclose an apparatus for solvent extraction followed by distillation. U.S. Pat. No. 2,966,450 (Kimberlin et al) also mentions the use of acetic acid in conjunction with solvent extraction apparatus, but indicates that acetic acid is unsuitable for this purpose, either because of ineffectiveness or because of lack of selectivity. The Gengembre patents and U.S. Pat. No. 2,073,367 (Fisher) disclose a rotating shaft with stirring fingers that agitate material inside a cylindrical vessel. Fisher relates to conversion of mixtures of hydrocarbon oil and solid or semi-solid bituminous material, such as coal, peat, lignite, oil shale and the like, for the production of gas and liquid products.
U.S. Pat. No. 4,260,471 (Miller) extracts sulfides from coal with acetic acid in the production of synthetic fuel. Sulfur, bonded in pyritic and organic compounds, is converted into a sulfide compound which may be extracted or leached from the coal or carbonaceous material with an acid. Extraction of the sulfides with an acidic compound results in the dissolution of the sulfide compounds and the release of sulfur as hydrogen sulfide. The hydrogen sulfide is then sent to a hydrogen sulfide absorbing solution, such as an amine solution, where hydrogen sulfide is recovered. In a preferred mode of the invention disclosed in U.S. Pat. No. 4,260,471 (Miller), crushed coal, water and acid are introduced into a mixing vessel to prepare a slurry. A wide variety of acids may be used, including hydrochloric, nitric and even acetic acid. The slurry is transferred to a heating zone and then a pressurized reaction zone where the pressure is increased to 100 to 4,000 p.s.i. for 0.5 to 30 hours, for removal of pyritic sulfur, although organic sulfur remains. There is, however, no mention of distillation.
U.S. Pat. No. 2,701,787 (Hemminger et al.) discloses an apparatus for distilling oil shale wherein the heat for distillation is supplied by carrying out the distillation in indirect heat exchange with a fluidized mass of spent shale highly heated by combustion.
U.S. Pat. Nos. 4,104,537 (McQuitty) and 4,107,029 (Lorenz) each disclose a scraper at the bottom of a cylindrical vessel adapted to treat aqueous suspension or slurries of tar sands or the like. U.S. Pat. Nos. 4,110,194 (Peterson et al.) and 4,189,376 (Mitchell) both disclose solvent extraction processes involving helical screw type conveyors for removing used particulate solids from the bottom of a treatment vessel. Peterson also discloses stirring fingers. Mitchell discloses means for injecting steam and solvent into different zones of the extraction vessel.
There remains a need for a more efficient, complete and economical extraction process in which a higher proportion of known valuable products are recovered. There also remains a need for a process which enables the recovery of new products, making the oil shale retort operation more economically sustainable.